It is common in electro-mechanical machinery for a rotating structure to be coupled to a radially internal or radially external static structure. It is often necessary to transfer fluids between the static and rotating structure. This can be achieved by providing openings in the rotating and static part of a coupling, when the openings are in alignment, fluid can be transferred across the coupling. A problem arising with this arrangement is the interruption of flow which occurs when the openings are out of alignment and the extreme variation in flow area as the openings come in and out of alignment.
One solution which has been adopted is the use of baffles within or around the couplings to buffer the flow. Whilst this solution can be effective, the additional complexity and weight of the solution renders it unattractive or impractical in some applications.